The present invention relates generally to an apparatus and method for measuring fluid volume, and more particularly, to an apparatus and method that measures the volume of a drop of fluid falling through free space.
Conventionally, in an intravenous (hereafter xe2x80x9cIVxe2x80x9d) infusion apparatus, an aqueous solution of medication in an inverted bottle, or other type of fluid reservoir, is supplied to a patient through a fluid administration set comprising tubing, a cannula for insertion into the patient""s blood vessel, various fluid control devices such as clamps, injection sites, and at the upstream end, a drip chamber device. The drip chamber device includes a drop former at its upstream end, a transparent chamber through which drops are to fall, and an outlet port at its downstream end. The drop former portion is typically constructed to form drops having a predetermined volume. It has become typical for a drip chamber manufacturer to specify that a particular number of drops equals a particular volume of fluid. For example, a drop former may be constructed such that 20 drops equals a milliliter. As the fluid is supplied to the drip chamber from the fluid reservoir, the drop former generates drops of the fluid that fall through the transparent chamber to the outlet port. The very existence of drops indicates that fluid is flowing in the fluid administration system. The falling drops can be visibly observed in the transparent chamber and counted over a unit of time to calculate the flow rate. The flow rate can be adjusted by a clamp or other device upstream of the drip chamber device, or by downstream means such as an infusion pump. If an infusion pump is used, it will engage the administration set downstream of the drip chamber device and can be used to set a flow rate. The frequency of drops through the drip chamber will then depend on the flow rate set at the infusion pump.
A need exists for a reliable system that can more accurately measure the rate of flow of infusion medication through a fluid administration set. Typically, a treatment fluid is prescribed at a particular flow rate for a patient. Staying at that flow rate is desired so that the prescribed treatment is delivered. In the case where a transparent drip chamber is used, an upstream valve or clamp can be adjusted to control the rate of flow. Drops are observed in the drip chamber and are counted in an effort to monitor that the prescribed flow rate has been set. However, there are many factors that tend to cause the rate of flow to change after it has been initially set. For example, the rate of drop formation is dependent on the head height/pressure of the fluid reservoir. Depletion of the fluid supply will decrease the head pressure on the drop former and will cause a diminution in the rate of drop formation and flow. Vibration or shock may cause the rate controlling clamp to change its adjustment. An obstruction may find its way into the drop former causing the formation of smaller drops thus changing the rate of fluid flow. Uneven pumping by a downstream infusion pump can also cause variances in the frequency and shape of the drops. It would be helpful to make it readily known when a change in the rate of flow has occurred so that restoration of the desired flow rate can be effected.
As another consideration, the flow rate is typically determined by counting the number of drops per unit time and then performing a calculation to determine the actual flow rate. For example, if forty drops are counted in a time period of one minute, and the specification of the drip chamber is that twenty drops equals one milliliter, then the calculation is that a flow rate of two milliliters per minute exists. Should the actual volume in a drop vary from that specified for the particular drip chamber device, the actual fluid flow rate to the patient may be different from that calculated. This would likewise be undesirable as the patient would not be receiving the prescribed fluid flow.
Monitoring the flow rate through visual observation of drops as described above usually requires personal monitoring of the infusion by a nurse or other medical personnel. Infusions typically extend over a long period of time and this need for monitoring therefore represents a considerable problem to hospital personnel, especially when nurses are in short supply. The need to time drops over several minutes to determine the flow rate may occupy a significant amount of a nurse""s time thereby leaving less time to perform other duties. A need to return numerous times during a lengthy infusion to again count drops also results in an increased demand on nurses whose schedules are already typically very busy.
Approaches for automating the monitoring process have been provided in the past. Many attempts have been made at providing an automated drop counter. While such systems have proved useful, they do not indicate by direct measurement the actual volume of the fluid detected. They only indicate that a drop has been detected. Such automated systems then use the drop volume as specified by the manufacturer of the drip chamber to determine volume. As discussed above, this may not always be accurate.
One prior approach is optical in nature and includes an array of photo detectors used to determine the size of the shadow of a drop as it passes in front of the detector. However, the variability of the optical qualities of drip chambers has posed a difficult obstacle to overcome. Also, condensation in the drip chamber can interfere with accuracy of an optical system as can intense room lighting. Another environmental condition that has impacted the usefulness of optical systems is the tilting of the drip chamber so severely that the drops may only partially pass across the photo detector. Further, the shape of drops varies from drop to drop. Those techniques that measure only one or two linear dimensions of a drop to determine its volume can have less than desirable accuracy due to this change in drop shape. It has been noted that a system that measures only one linear dimension of a drop, such as only length, to determine the drop volume can miscalculate the volume of the drop by thirty percent or more. Other optical methods have also been less accurate than desirable due to one or more of the above reasons.
Another method, as disclosed in U.S. Pat. No. 4,583,975 to Pekkarinen et al., is based on the piezoelectric effect. This system includes a piezoelectric film mounted to the inner wall of a drip chamber beneath the surface of accumulated fluid in the chamber. As a drop impinges on the surface of the accumulated fluid, the piezoelectric element is stressed and a voltage differential signal is generated. The method comprises the direct contact of an electric element of an electric circuit with the accumulated fluid in a drip chamber; fluid that may be in direct contact with a patient during an infusion thereby making this an undesirable approach for multiple reasons.
Another technique uses capacitive-based sensors to approximately determine the rate of fluid flow within a drip chamber. However, these methods are used only to detect the existence of a drop. They allow determination of the frequency of the drops, and rely upon the manufacturer""s specified drop volume and an assumed constant drop volume to calculate the rate of flow. They do not take measurements sufficient to allow the actual volume of a drop itself to be determined. Further examples of capacitive-based sensors found in the prior art take measurements that allow determination of the accumulated fluid level in a drip chamber, from which flow rates may be determined, but do not take measurements from which the actual volume of a drop itself may be determined.
Although increased accuracy in fluid flow measurement is desired, cost is always a concern. The ability to make better health care available to an increasing number of people demands that the costs be kept as low as possible. It is desirable to lower the costs of medical devices so that they are affordable to a greater number of people.
Hence those skilled in the art have recognized a need for a fluid flow measurement device that is more accurate. A need has also been recognized for a flow sensing system that is insensitive to the shape of drops and is also insensitive to the optical characteristics of those drops. There has also been recognized a need to determine the volume of drops in a drip chamber so that an actual flow rate can be more accurately monitored. There is also a need for such a flow measurement device that is easier to manufacture and easier to use. The present invention fulfills these needs and others.
Briefly, and in general terms, the present invention is directed to a capacitive-based sensor for measuring the volume of a fluid drop passing through an IV drip chamber. The sensor includes a capacitor comprising two parallel plates which are a fixed distance apart and are positioned on either side of a drip chamber. The plates are positioned such that a fluid drop falls through the space between the plates, thereby causing the capacitance provided by the plates to change. This change in capacitance is measured and based on that sensed change in capacitance, the volume of the drop is more accurately calculated.
In one aspect, the change in capacitance is measured by incorporating the parallel plates into a resonant circuit having a resonant frequency dependent on the capacitance of the parallel plates. Any change in the resonant frequency of the circuit, as would be induced by a change in the capacitance of the parallel plates is detected and measured. The capacitance change is determined from the frequency change in the resonant circuit. In another aspect in accordance with the invention, the parallel plates may be part of a capacitive balanced bridge circuit, which experiences a change in capacitance when a fluid drop falls between the plates.
In both of the above-described aspects, the change in capacitance of the plates resulting from a drop falling between them is used to calculate the volume of the drop. The method of the present invention has the advantage of directly measuring the volume of each drop, thus eliminating the disadvantage of having to assume a drop size or shape. Moreover, the optical qualities of the drip chamber have no effect on the determination of the drop volume, thus eliminating a troublesome aspect of prior art methods which rely on optical methods to calculate drop size.
In another aspect, the present invention includes an electronic circuit which records the measured volume of each drop of a series of drops falling through an IV drip chamber over a period of time and integrates the result, thereby permitting measurement of the fluid flow rate through the drip chamber. In a further aspect, the present invention includes an electronic circuit which enables the device of the present invention to adjust the actual rate of flow through the drip chamber, based on the measured rate of flow.
Other features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.